exports.done = false;
console.log('.....a1.....')
var b = require('./b.js')
console.log('.....a2.....')
console.log('在 a.js 之中，b.done = %j', b.done)
console.log('.....a3.....')
exports.done = true;
console.log('.....a4.....')
console.log('a.js 执行完毕');